P405: IN VITRO RESPONSE TO BCL-2 INHIBITION IN PEDIATRIC B PRECURSOR ALL WITH HIGH RISK CYTOGENETICS
Authors: Francesca Gottardi, Daria Messelodi, Domenico D’amico, Francesco Baccelli, Davide Leardini, Salvatore Nicola Bertuccio, Riccardo Masetti
Journal: HemaSphere (2023)

Background: BCL-2 inhibitor venetoclax is approved for chronic lymphocytic leukemia and acute myeloid leukemia, with increase evidence of efficacy and safety also in pediatric myeloid malignacies. Its role in pediatric acute lymphoblastic leukemia (ALL) is still under debate and clinical evidences are limited. Increasing preclinical evidence of sensitivity to BCL-2 inhibition has been reported for specific high risk cytogenetic B-ALL categories. In KMT2A-AFF1, TCF3-HLF and hypodiploid B-ALL, in vitro sensitivity to BCL-2 specific inhibition and in vivo efficacy of venetoclax in patient-derived xenografts (PDX) have been documented. Clinical activity of the combination venetoclax/navitoclax has been documented in clinical trials in ALL, with reported activity in hypodiploid cases. Documented clinical activity for KMT2-AFF1 ALL and TCF3-HLF lacks. Aims: The principal aim of this work is to perform a preliminary in vitro analysis of Bcl-2 inhibition in high risk cytogenetics pediatric ALL. A secondary aim is to explore a potential efficacy of early introduction of venetoclax in treatment in case of failure of first line approaches. Methods: We analyzed samples isolated from four patients with high-risk cytogenetic B-ALL diagnosed at our Centre: two KMT2A-AFF1 (one first diagnosis and one relapsed), one TCF3-HLF and one hypodiploid B-ALL. Only bone narrow (BM) samples containing more than 50% of blasts infiltration were selected for response evaluation. Cells were treated with ABT-199 (Venetoclax) for 72 hours with a concentration scale ranging from 10µM to 2nM, in parallel with healthy donor BM cells as control. Cell death rate was evaluated through MTT assay. Gene expression analysis by RT-PCR of BCL-2, BCL-XL and MCL-1 was tested in each sample. Results: Except for hypodiploid ALL, which was not evaluable for low blasts percentage, all other samples showed a good in vitro response, with significative death rate from low molar ABT-199 concentrations (Fig.1A). Response was remarkably strong in patient with first diagnosis of KMT2A-AFF1 ALL. Gene expression analysis revealed high BCL-2 expression in all patients, extremely high BCL-XL expression in TCF3-HLF and hypodiploid patients, and down-regulation of MCL-1 in all samples (Fig. 1B). Based on these results, patient with TCF3-HLF ALL was treated on compassionate use with venetoclax and navitoclax added to Inotuzumab Ozogamicin achieving a complete response on both BM (negative minimal residual disease) and extra-medullary bone lesions (Fig 1C, submitted on 11/02/2023) after primary induction failure to conventional therapy. Summary/Conclusion: Our findings further support preclinical evidence about sensitivity to BCL-2 inhibition of high risk cytogenetics B-ALL. A deeper characterization including BH3 profiling and gene expression profile are needed to elucidate these findings. Clinical trials are currently ongoing exploring venetoclax activity in children with B-ALL. Characterization and selection of potential responders may be useful for early introduction of venetoclax in relapsed patients.Keywords: B cell acute lymphoblastic leukemia, Venetoclax, BCL2, Children